fix: handle camel case package

[cobbinma]

Issue 51

convert package name to snake case when writing file to be consistent with prost_build 🐍

[tustvold]

LGTM, thank you 👍

I'm guessing this just impacts the name of the files, and not anything else?

Also to clarify this is technically a breaking change, as whilst not consistent with prost_build, it still did work for such packages?

[cobbinma]

Yes, this should only impact the file names.

I am generating a mod.rs file after using tonic_build and pbjson_build. The json implementations are placed in the same module based on the file name.

pub mod google {
    pub mod pubsub {
        pub mod v1 {
           include!("google.pubsub.v1.rs")
           include!("google.pubsub.v1.serde.rs")
        }
    }
}

If the file names are different from tonic_build the approach will not work as both files need to be in the same module.

I could probably use the a custom writer to get around this but thought this was better default behaviour.